The present invention relates generally to the field of remote detection. Specifically, the present invention relates to passive millimeter wave spectroscopy for remote detection.
Remote detection systems have become increasingly important in helping to regulate behavior of individuals and corporations. Remote sensing devices serve a wide range of functions from detecting contamination to determining if a person is carrying contraband. Some situations require that a remote sensing device be able to function over a large distance rather than over a short-range only.
Active and passive spectrometers operating in the optical range have been employed in the past for a variety of environmental monitoring applications. Optical techniques are commonly used for remote sensing. However, optical systems, which provide superb sensitivity as a direct consequence of high-energy vibrational transitions, are limited in their scope of application because of their extreme sensitivity to atmospheric effects and inherent range limitations. Remote sensing of terrestrial targets based on radar backscattering at microwave frequencies are routinely used to monitor temporal variations of the earth's surface. Short-range spectroscopic measurement of molecular absorptions at millimeter wave frequencies have also been studied extensively in the past, primarily as laboratory-based techniques. Passive microwave sensing and imaging of materials have also been applied in the past for special applications, including the measurement of thicknesses for dielectric medium and short-range determination of thermal and chemical signatures.
Because of the thermal interaction at atomic and moleculecular level, all materials radiate electromagnetic (EM) energy. A radiometer, which essentially is a highly sensitive receiver, can be used to detect blackbody radiation over a narrow range of the EM spectrum. With the radiation spectrum being governed by Planck's radiation law, the sensor output is a measure of the temperature of the scene; it varies nearly linearly at millimeter wavelengths. Passive sensing of EM radiation at microwave frequencies has been used in the past primarily for radioastronomical and atmospheric observations for which the sensor antenna is directed away from the surface of the earth.
Radio frequency (RF) radiometers are generally designed to operate within atmospheric windows in the microwave and low millimeter-wave (MMW) bands where attenuation due to highly absorbing molecules in the atmosphere is lower. Millimeter waves are radio waves sent at terahertz frequencies, known as terahertz radiation, terahertz waves, T-rays, T-light, T-lux and THz, are in the region of the light spectrum between 10 terahertz and 100 gigahertz, corresponding to the wavelength range 30 micrometers (ending edge of far-infrared light, micrometer wavelength) to 3 mm (starting edge of microwave radiation, millimeter wavelength).
The primary application of passive MMW remote sensing has so far been dedicated to satellite and high-altitude observation of the upper atmosphere. However, there is a need for application of spectroscopic techniques for passive terrestrial detection of target molecules in a chemical plume at millimeter wave frequencies.